When cells divide, organelles and macromolecules can be partitioned unequally between daughters. This process of asymmetric partitioning during division is central to cellular differentiation and the long range goal of this study is to understand its genetic control. We study the genetic control of asymmetric partitioning during Caenorhabditis elegans spermatocyte development, which offers some of the advantages of a unicellular system within a metazoan organism. C. elegans primary spermatocytes will develop into spermatids in vitro in simple, chemically defined media that lacks added growth factors, hormones or accessory cells. Many mutants specifically disrupt the distinctive asymmetric partitioning required for C. elegans spermatocytes to properly divide and differentiate into spermatids. Such spermatogenesis-defective (spe) mutants affect a specific, cytologically obvious organelle that can be easily followed during spermatocyte differentiation. These organelles, the ER/Golgi derived fibrous body-membranous organelles (FB-MOs), are key players in asymmetric partitioning and they do not function properly in the spe mutants that are the subject of this proposal. The mutants to be studied fall into two distinctive phenotypic classes. Class I mutants show cytologically evident defects in FB-MO morphology and proper asymmetric partitioning does not occur because of these defects. Class II mutants initiate FB-MO morphogenesis normally, but fail to properly position these organelles because the machinery required for proper asymmetric partitioning is defective. These proposed analyses of Class I and Class II mutants will reveal the mechanism of asymmetric partitioning and how FB-MOs participate in this important process. This analysis will test the hypothesis that specific gene products actively segregate the FB-MOs during morphogenesis. The discovery that wild type FB-MOs contain the SPE-4 protein (a homolog of the Alzheimer's disease-causing presenilins), and that this protein is missing in the Class I mutant spe-4, has heightened the significance of and interest in this organelle. The role of presenilin homologs during asymmetric partitioning can only be determined in a metazoan system and the single-celled nature of C. elegans spermatocytes facilitates cell biological analyses. Asymmetric partitioning is a generally important problem that will provide insights into the pathologic changes occurring during diseases such as cancer and Alzheimer's disease. Additionally, C. elegans spermatogenesis is the best model for this process in the many medically and agriculturally important parasitic nematodes that are difficult to study in the laboratory.